1. Field of the Invention
The present invention relates to a permanently-charged electret toner for use in developers for developing electrostatic latent images formed in electrophotography, electrostatic recording and electrostatic printing.
2. Discussion of the Background
Conventional developers for use in electrophotography, electrostatic recording and electrostatic printing, in their developing processes, are transferred to image bearers such as photoreceptors electrostatic latent images are formed on, transferred therefrom to a transfer medium such as a transfer paper, and fixed thereon.
As the developers for developing electrostatic latent images formed on image bearers, a two-component developer including a carrier and a toner, and a one-component developer without a carrier, such as a magnetic toner and a nonmagnetic toner, are known.
As a dry toner for use in electrophotography, electrostatic recording and electrostatic printing, a pulverized toner is widely used, which is formed by kneading a toner binder such as styrene resins and polyester resins with a colorant, etc. upon application of heat, cooling the kneaded mixture to be solidified and pulverizing the solidified mixture.
Recently, polymerized toners prepared by suspension polymerization methods, emulsion polymerization condensation methods, etc. are being used.
In addition, Japanese Laid-Open Patent Publication No. 7-152202 discloses a polymer solution suspension method of using a volume contraction.
This method includes dispersing or dissolving toner constituents in a volatile solvent such as an organic solvent having a low boiling point to prepare a dispersion or a solution, emulsifying the dispersion or solution in an aqueous medium to form a droplet, and removing the volatile solvent.
The diversity of resins this method can use is wider than those of the suspension polymerization methods and emulsion polymerization condensation methods, and has an advantage of being capable of using a polyester resin effectively used for full-color images requiring transparency and smoothness.
However, conventional toners need a frictional charge with charging members such as a carrier, a developing sleeve and a developing doctor to be charged as desired.
As a charging method without a frictional charge, a method of injecting a charge to an electroconductive toner is disclosed, however, this cannot be applied to an insulating toner due to its poor charge injection efficiency.
In addition, the application of the electroconductive toner is limited in electrophotography because of its unstable chargeability and poor transferability.
Further, Japanese Laid-Open Patent Publication No. 6-308815 discloses a developing method without a frictional charge using an electret toner, however, the productivity thereof is low.
Because of these reasons, a need exists for a permanently-charged electret toner needing no complicated and costly toner charging mechanism or agitator, being uniformly charged without uneven charge quantity, having good storage stability and producing images faithful to latent images for long periods.